(1) Field of the Invention
The present invention relates generally to semiconductor integrated circuit processing and more particularly to gate dielectric layers of MOSFET devices.
(2) Description of Prior Art
The gate dielectric is perhaps the most sensitive component of modern CMOS devices. Requirements of appropriate dielectric layers include high dielectric breakdown strength, good diffusion barrier properties, low trapping densities and low interfacial states. As device dimensions decrease there is a corresponding need for thinner gate dielectric layers. Generally, requirements become more stringent and new thinner gate dielectric structures are needed that meet the more stringent requirements.
Silicon oxynitride layers are used for gate dielectrics in various CMOS technologies and it is expected that their use will extend into the near future. To optimize the performance of silicon oxynitride layers, which are essentially gate oxide layers containing nitrogen, the amount and distribution of the nitrogen in the layers is of utmost importance. It is imperative to have a large concentration of nitrogen near the gate/dielectric interface to act as a penetration barrier. For instance, in PMOS a large concentration of nitrogen in the gate dielectric layer at the interface with the polysilicon gate is required to prevent boron penetration. Nitrogen is also required in the dielectric at the dielectric/silicon interface for increased resistance to hot carrier affects and for improved interface quality. At the dielectric/silicon interface however, the nitrogen concentration needs to be limited since too much nitrogen is detrimental, giving rise to decreased channel carrier mobility and degraded reliability. A bimodal nitrogen concentration profile in the dielectric would be ideal, with the highest concentration near the gate/dielectric interface, a lower but still high concentration near the dielectric/silicon interface and a moderate concentration in between. Preferred embodiments of the invention provide improved methods for achieving such a profile.
Tuning the nitrogen concentration formula to achieve improved performance is at times denoted Nitrogen profile engineering. Several approaches have been proposed. Gusev et al., J. Appl. Phys. 84, pages 2980–2982, 1998, present a method for tuning the nitrogen concentration profile utilizing thermal processing. Their approach involves using NO/O2/NO gasses sequentially. Dang and Takoudis, J. Appl. Phys. 86, pages 1326–1330, 1999, extended the method. They studied the affect on the nitrogen concentration profile of different process sequences using NO, N2O and O2 gasses. The technique of Dang and Takoudis relies on the different thermal nitridation characteristics of the different nitrogen sources, i.e. NO and N2O gasses. These methods are limited, however, in that thermal nitridation is difficult to extend to the sub 1.6 nm EOT range and these methods cannot fine-tune the nitrogen concentration profile to achieve an optimized bimodal nitrogen concentration profile for such thin dielectric layers.
Other nitrogen profiling techniques utilize plasma nitridation. Kraft et al., U.S. Pat. No. 6,136,654, provide a plasma nitridation method for introducing non-uniform concentrations of nitrogen that is incorporated into an oxide layer or forms a nitride layer at the surface of the substrate. As pointed out by Nimi et al., U.S. Pat. No. 6,610,614, the method is not suitable for oxide layers less than about 2 nm thick. Nimi et al. also use plasma nitridation to introduce nitrogen into an oxide film but add re-oxidation and annealing steps to stabilize the nitrogen concentration profile, heal plasma induced damage and reduce interfacial defect densities. McFadden et al., U.S. Pat. No. 6,610,615 discloses a low power direct plasma method for introducing nitrogen into an oxide layer. Using a gas having lower ionization energy than nitrogen in combination with nitrogen results in a steeper concentration profile for nitrogen in the oxide layer. However, none of the above patents, McFadden et al., Kraft et al. or Nimi et al., provide a method to achieve an optimized bimodal nitrogen concentration profile for thin dielectric layers.
Shue et al., U.S. Pat. No. 6,380,056, disclose forming a silicon layer, exposing the silicon layer to nitrogen containing annealing atmosphere to form a silicon nitride layer, then oxidizing to form a silicon oxynitride layer. U.S. Pat. No. 5,563,093, to Koda et al., and U.S. Pat. No. 5,597,754, to Lou et al., teach methods of forming silicon gates utilizing Si2H6. U.S. Patent Application 2004/0145029 to Acetutu et al. shows a silicon oxynitride antireflective coating layer formed using Si2H6 or Si3H8, for example as silicon source. U.S. Patent Application 2002/0009900 to Tay et al. discloses a silicon oxynitride layer formed on silicon by rapid thermal processing.
Conventional process flows used to tune the nitrogen concentration profile in dielectric layers are shown in FIG. 1. Region 2 is a silicon region, which could be a silicon substrate or a silicon region formed on a substrate. An oxide layer, 4, is formed over the surface of the silicon region. Nitrogen is introduced through the surface of the oxide layer, 4, from nitrogen containing species, 6, in the atmosphere above the oxide layer. This can be done by thermal nitridation, which, as pointed out by Nimi et al., U.S. Pat. No. 6,610,614, seems to be unsuitable to modern ultra-thin oxide layers. In the case of thermal nitridation the nitrogen concentration profile tuning is accomplished by the choice of nitrogen sources, such as NO or N2O, the process sequence, in which other gases, such as O2, can be included, and the conditions of annealing steps. Plasma nitridation has been studied as an alternative to thermal nitridation, particularly as a method of introducing large concentrations of nitrogen localized in the oxide layer at the gate-dielectric interface. Tuning the nitrogen concentration profile by alteration of the plasma parameters has proven problematic, however, due to contradictory behavior in the nitridation process for different nitrogen containing species. For example, nitrogen ions in the plasma do not have high diffusivity, but usually have high kinetic energy. Nitrogen radicals, usually do not high kinetic energy, but have high diffusivity. Thus compromises have to be made in terms of the choice of plasma parameters to achieve nitrogen concentration profile tuning. Some improvements have been made in terms of plasma source control, such as, for instance, using pulsed RF sources and the addition of helium to the plasma, that improve the tenability. However there is still much room for improvement, for which it is a primary objective of the invention to provide.